The present invention relates to a formable filament nonwoven fabric which is biodegradable by microorganisms and the like in natural environments, and a method of producing the same.
Hitherto, nonwoven fabrics which are degradable by microorganisms have been known, examples thereof including degradable nonwoven fabrics composed of natural or regenerated fibers or filaments such as of cotton, flax, hemp, ramie, wool, rayon, chitin and alginic acid.
However, such degradable nonwoven fabrics, which are generally hydrophilic and water absorptive, are not suitable for use in such an application as disposable diaper top sheet which should have hydrophobic and less water absorptive properties and provide a dry tactile feeling even in a wet or moistened state. These nonwoven fabrics are much liable to deterioration in strength and dimensional stability under wet and moistened environmental conditions and, hence, find limited application in the general industrial material field. Further, these nonwoven fabrics are not thermoformable because of their non-thermoplastic property and, hence, are inferior in processability.
Therefore, positive research and development have recently been made on microbially degradable filaments which are obtainable by the melt spinning technique from microbially degradable polymers having thermoplastic and hydrophobic properties, and on microbially degradable nonwoven fabrics composed of such filaments. In particular, a group of polymers generally called aliphatic polyesters are attracting high attention because they are microbially degradable. Specific examples of such polymers include poly-xcex2-hydroxyalkanoates as typical microbially degradable polyesters, poly-xcfx89-hydroxyalkanoates such as polycaprolactone, polyalkylene dicarboxylates such as polybutylene succinate which are polycondensates of a glycol and a dicarboxylic acid, and copolymers of these polymers. In recent development of a new polymerization process which ensures efficient production of polymers of high polymerization degree, various attempts have been made to produce filaments from poly-xcex1-hydroxyacids such as poly-L-lactic acid and nonwoven fabrics composed of such filaments. Of the aforesaid aliphatic polyesters, polylactic acid in particular has a relatively high melting point, so that nonwoven fabrics composed of polylactic acid filaments are possibly useful in applications which require heat resistance. Therefore, much expectation is now directed toward practical application of the polylactic acid nonwoven fabrics.
A variety of products, such as disposable food trays, bottles and pots, molded or formed from these biodegradable polymers through an injection molding process, a blow molding process, a compression thermoforming process or the like have been under study, and some of these products have been put into practical use.
Although the products molded or formed through these processes are rigid and superior in shape retaining property, the processes are not suitable for production of filters, plant pots and the like which require air permeability and liquid permeability, nor for production of shoulder pads, masks, caps, hats and the like which require fibrous texture and softness. In the compression thermoforming process, a sheet material is employed which has a high stiffness, so that it is difficult to efficiently form the sheet material into a complicated configuration. In addition, the products thus molded or formed have smaller specific surface areas per unit polymer weight and, hence, have low biodegradation rates in composting.
It is an object of the present invention to provide a formable biodegradable filament nonwoven fabric which can be formed into a complicated configuration and composted in an extremely short time, and is applicable to products which require air permeability and liquid permeability.
In accordance with one aspect of the present invention to accomplish this object, there is provided a thermoformable nonwoven fabric composed of filaments of a biodegradable polymer which comprises a thermoplastic aliphatic polyester as its principal component, wherein the filaments have a polymer supercool index of 0.3 to 0.6.
In accordance with another aspect of the present invention, there is provided a method of producing a thermoformable nonwoven fabric composed of filaments of a biodegradable polymer comprising a thermoplastic aliphatic polyester as its principal component, the method comprising the steps of: melting the polymer and extruding the resulting melt through a spinneret into filaments; drafting the extruded filaments at a drafting speed of 1,000 to 2,500 m/min by means of a suction device disposed below the spinneret, while quenching the filaments with quench air blow; spreading and accumulating the drafted filaments on a movable collector surface thereby to form a web; and treating the web for formation of the nonwoven fabric.
In accordance with the present invention, the nonwoven fabric is formed of the filaments which are obtained by drafting the extruded filaments at a drafting speed of 1,000 to 2,500 m/min under quenching below the spinneret and thereby have a polymer supercool index of 0.3 to 0.6. Therefore, the nonwoven fabric maintains a sufficient filament mobility during a thermoforming process, particularly, when subjected to a complicated acute-angle forming process. In addition, the nonwoven fabric has a low distortion stress and a high breaking extension at a thermoforming temperature. Therefore, breakage of the nonwoven fabric is effectively prevented, which may otherwise occur during the thermoforming process. Further, products thermoformed from the filament nonwoven fabric have a larger specific surface area than products thermoformed from conventional sheet materials and, therefore, can be composted in an extremely short time. Further, the filament nonwoven fabric of the present invention can be formed into products which require air permeability and liquid permeability and products which require fibrous or nonwoven texture.